High costs of aircraft operation and desires on the part of governmental agencies to efficiently and effectively train aircraft pilots have resulted in efforts to develop cost effective flight simulator apparatus. The United States Federal Aviation Administration has recently implemented an advanced flight simulation program requiring that flight simulators accurately depict an aircraft's performance on take-off and landing maneuvers as well as on the ground, provide an improved visual response time and increased fields of vision, and provide daylight capabilities plus adverse weather features to allow pilots to upgrade from copilot to captain on the same aircraft or to laterally transfer crew members from one aircraft type to another entirely in a simulator.
With operating costs of large jet liners such as a Boeing 747 running $6,500 to $7,000 per hour, commercial carriers and the military alike are interested in lower cost flight simulators for providing total simulation training. Flight simulation apparatus employing high-speed supercomputers and superminicomputers to create graphics displays in real time have been developed in response to this need. Costs of many current flight simulation apparatus are around $250-300 per hour due to the high computational cost of generating real-time images for the flight simulators which can be reproduced rapidly enough to be convincingly realistic. Costs of this order, however, are prohibitive for smaller commercial and private carriers and private pilots, who must still rely upon actual flight time for training and upgrading. Moreover, the resolution of the graphics displays of many of these systems leaves much to be desired when the computer-generated images are created at rates high enough to avoid perceptible flicker. Accordingly, there is a need for a means of producing convincingly realistic images for flight simulators at drastically reduced costs which do not involve the real-time generation of sophisticated realistic computer graphics by supercomputers and superminicomputers.
Recent advances in generating realistic graphic images by computer have made it desirable to incorporate these new techniques into flight simulation. Although costs of the computers and software for generating realistic computer images are falling rapidly, the computational demands of realistic image creation are immense. Typically, supercomputers such as a Cray X-MP Super Computer, manufactured by Cray Research, can perform over 400 million mathematical computations per second, but even such a supercomputer can produce only around 25 minutes of high quality 70-millimeter computer-generated film images per month. Fast minicomputers can produce an average of only about 2.5 minutes of 70-millimeter film images per year. While it is desirable to use newer image synthesis techniques such as fractal geometry for generating images for flight simulation and other applications such as games, it is presently difficult if not impossible to generate and display such images in real time. Accordingly, there is a need for a method of creating highly realistic images off-line, and providing for storage and retrieval of previously-generated images for applications such as flight simulators.
One prior art technique for generating images for use in simulators is known as texture mapping. This technique has been successfully employed by the Jet Propulsion Laboratory (JPL), Pasadena, Calif., to generate simulated images for the planning stages of the Voyager spacecraft in its recent fly-by of the planet Jupiter. Two-dimensional images from paintings and photographs were remapped to a sphere so that various planet fly-by scenarios could be simulated for the space vehicle.
In a texture mapping system, an image that is projected on one surface is remapped to a different surface by ray tracing. For example, one surface which represents the terrain that is to be simulated may be a plane, like an overhead map or satellite photograph. This first surface is remapped to some other surface; this may only involve remapping to another plane to effect perspective transformation. The other plane could be a CRT display screen in a vehicle simulator--the data from the stored representation of the terrain in the image data base is mapped to the plane of the screen which is in the simulator. As another example, for the JPL Jupiter simulation, and in many recent popular science fiction movies, there is a mapping to a spherical representation to simulate a spinning globe of a planet. Of course, these are non-real-time simulations.
There are several problems encountered in texture mapping approaches to simulation which make it expensive for real-time simulation. One significant problem is that computations can be very complicated and therefore computationally expensive. Another major complication involves sampling artifacts or aliasing, especially on complicated surfaces such as remapping from a plane to a sphere. One aspect of the problem results from uneven resolutions between the mapped surface and the mapping surface. Some areas may have a high resolution on the original surface while other areas have very coarse resolution. For example, consider the remapping of a regular Mercator projection map to a globe--on the Mercator projection the data at poles has a very high resolution. When the data is mapped to the globe's surface, all the data gets condensed. When this is done pixel-by-pixel, as in a digital imaging system, some of this condensed data may be skipped over and cause the effect often called "aliasing" or "sparkling". In the case of moving sequences in particular, there can be irregular edges or possibly holes in the images. Other effects of texture mapping include the situation where the original resolution was insufficient (such as on the equator) and the data at certain regions (on the equator) becomes coarse or blocky when remapped because the pixels are becoming very large relative to the new projection surface. In real-time simulators, aliasing is unacceptable because it severely degrades the realism of the simulated images.